The three-dimensional structure of the lysozyme from bacteriophage T4 has been determined by X-ray crystallography from a 2.5 A resolution electron density map. Also an extensive library of mutant enzymes is available, and techniques for obtaining additional mutants have been established. We propose to compare in detail by X-ray crystallography, nuclear magnetic resonance, and various optical methods the three dimensional structures, the stability and the activity of selected mutant enzymes with that of the wild type enzyme. In so doing we expect to obtain detailed information, not previously accessible, concerning the factors influencing the stability and activity of T4 phage lysozyme, and by inference, of proteins in general. We also propose to compare, insofar as possible, the structure and activity of T4 phage lysozyme with that of hen egg-white lysozyme.